In recent years, along with practical use of WDM (Wavelength Division Multiplexing), application of this WDM to a network with a ring or mesh configuration has been expanded. In such network, an optical signal with an arbitrary wavelength is branched or inserted from a transmission path fiber using technologies, such as ROADM (Reconfigurable Optical Add Drop Multiplexing) and WXC (Wavelength Crossconnect), and the network is characterized in that a termination point does not exist as in a Point-to-Point network.
In addition, in the above-described ROADM, WXC, etc., AGC (Automatic Gain Control) control using input and output signals of an optical amplifier is performed (for example, refer to Patent Literatures 1, 2, and 3).